The fuel rods in use in nuclear reactors commonly use cylindrical pellets composed of uranium dioxide bound in a matrix material. Following grinding the pellets to their final shape and before they are stacked in the fuel rods, the pellets must be inspected for flaws and other anomalies, for conformance to predetermined length measurement and conformance to a cylindrical shape.
In use, the pellets are packed tightly in metallic tubes in the nuclear reactor. The metallic tubes dissipate heat generated by the pellets into a surrounding medium. Thus, a good heat-exchange relationship must exist at the pellet-metallic tubing interface and this is provided when the pellets' surfaces conform to the cylindrical shape of the tubing. Inspection for conformance to this and other criteria is conventionally carried out by automated optical equipment which examines pellets and sorts them according to their degree of conformance.
One approach used in prior art apparatus for optically inspecting such pellets employs mechanical hands to carry individual pellets from a stream of pellets to an inspection station. At the inspection station each pellet is rotated in order to completely expose it to view and other mechanical hands transport it back to the pellet stream where sorting is carried out to deliver the pellets to selected locations.
The prior art apparatus described requires complicated mechanical linkages to operate the hands, and the speed of inspection is limited by the speed with which the hands can move the pellet out of and back into the pellet stream, respectively. Further, since the pellets are composed of a highly abrasive material, the roller supports on which the pellets are rotated are rapidly abraded away, and the useful lifetime of each is limited. Often the rollers are abraded unevenly the develop surface ridges. These ridges can cause the pellets to jitter or chatter during rotation, thereby rendering the inspection equipment incapable of obtaining a clear pellet image and hence incapable of an accurate measurement. Such apparatus is also prone to problems of pellet identification. It has been found difficult to correlate information concerning the actual physical location of a particular pellet with the measured information concerning the same pellet obtained by the inspection equipment. Such a situation produces difficulties in coordinating the mechanical hands with the rest of the transport, thus causing further delays in pellet viewing and sorting.
Another approach used by the prior art for optically inspecting pellets employs a camera which moves along a track located above rollers that support a rotating string of pellets. Such apparatus also causes the rollers to wear unevenly, thus causing eventual unacceptably large vibrations and jitter of the rotating pellets.
Both types of equipment discussed above are prone to introduce dust into the air and to create a dust-laden atmosphere within which both types of inspection devices are required to function. The dust particles lodged in the mechanical linkages of either the moving camera or the mechanical hands are apt to cause vibrations and other objectional phenomena.